Asteroid Bennu: A Journey Through Solar System History | NASA

Comets and meteorites deliver prebiotic molecules to planets through impacts. The Stardust mission found amino acids in comet Wild 2, demonstrating that comets contain life's building blocks. This suggests comets brought both destruction and the chemical 'seeds' necessary for life. Water on Earth was likely delivered by comet impacts. Prebiotic chemistry can occur in three main environments: the atmosphere (Miller-Urey experiment), hydrothermal vents (high pressure and temperature alter reaction constants), and mineral surfaces (wet-dry cycles on clay can promote peptide bond formation). The hydrothermal vent hypothesis suggests life may have originated at ocean bottoms where material from Earth's mantle is released.

The molecular cloud from which our solar system formed contained virtually all the chemical building blocks that terrestrial biology uses: water, organic carbon compounds, amino acids, nucleobases, and sugars. The full chemical precursor inventory for building RNA, proteins, and cell membranes was present in the interstellar medium before the solar system formed. This prebiotic chemistry is not a special feature of our molecular cloud but a generic feature of cold interstellar chemistry that operates wherever molecular clouds form—meaning wherever stars and planetary systems form, they are being seeded with the same chemical inventory. The pathway from molecular cloud chemistry to early Earth's surface is delivery by small bodies—comets and carbonaceous asteroids—during the heavy bombardment phase (roughly 3.9 to 4.1 billion years ago). The total mass delivered to Earth during this period is estimated at 10^19 to 10^20 kg, comparable to the mass of a small asteroid belt. The same comets and asteroids that brought deuterium-enriched water also brought amino acids, nucleobases, and sugars. The key question is survival: do these organic compounds survive impact or are they destroyed? The answer depends on impact velocity and size—smaller impactors at lower velocities or arriving at shallow angles can deliver organic material intact, while micrometeorites (millimeter-scale and smaller particles) are the most efficient carriers of intact organic chemistry. If the origin of life requires building blocks that must be synthesized specifically on planetary surfaces under specific conditions, then the origin of life is constrained by planetary conditions. Only planets with the right atmosphere, energy sources, mineral catalysts, and chemistry will produce building blocks. However, if building blocks are delivered from space pre-made, universally distributed, arriving in every planetary system that forms from a molecular cloud, then the building block synthesis step is removed from the constraint list. Every habitable planet that forms within a molecular cloud gets the building blocks. The origin of life is constrained only by what happens after delivery—the organization of building blocks into functional living chemistry. This is a thermodynamic and kinetic process, not a random miracle. Given sufficient time, chemical diversity, energy flow, and reaction sites, the exploration of chemical space leading to the first self-replicating system becomes the statistically expected outcome of a long chemical process.

For prebiotic chemistry to work efficiently, organic molecules must be concentrated in local environments rather than dispersed in vast oceans. Low-velocity comet impacts create craters that serve as natural concentration chambers. When comets deliver hydrogen cyanide at moderate speeds (~below 15 km/s), resulting crater ponds provide ideal conditions for prebiotic chemistry. These ponds undergo wetting and drying cycles that concentrate salts and organic molecules while enabling chemical reactions. This mechanism solves the concentration problem that plagues other delivery scenarios, making it plausible that just a few low-velocity impacts could initiate the chemical networks leading to life.

This segment explores the chemical processes that led to life's emergence. The speaker explains prebiotic chemistry as the study of how organic molecules and life's building blocks (carbon, nitrogen, phosphorus, oxygen) came together before living organisms existed. He describes how water on Earth originated from volcanic activity and planetary cooling, with oceans forming around 4.2 billion years ago. The first life appeared as bacteria approximately 3.5-3.55 billion years ago, representing a relatively brief period after Earth's formation. This period of chemical evolution set the stage for biological life to emerge.

Billions of years ago, repeated asteroid impacts on early Earth created temporary reducing atmospheres rich in hydrogen, which enabled the formation of organic molecules like amino acids and nucleobases—the fundamental building blocks of life—despite the violent bombardment that initially made the planet seem uninhabitable.

Near-Earth Objects (NEOs) are asteroids, comets, and other celestial bodies whose orbits bring them close to Earth. Asteroids are rocky remnants of the early solar system, mostly found in the asteroid belt between Mars and Jupiter, while comets originate from the outer solar system and are composed of ice, dust, and rocky material. Space agencies like NASA and ESA have developed extensive programs to track and detect NEOs using telescopes worldwide, including the Widefield Infrared Survey Explorer (WISE) and the NEOWISE mission. Artificial intelligence has revolutionized detection—in May 2024, the AI-based algorithm THOR detected over 27,000 previously unobserved asteroids by processing 1.7 billion data points per image. The Vera Cúria Observatory will further enhance detection capabilities using advanced AI software. Scientists have developed planetary defense strategies including kinetic impact (tested by NASA's DART mission), gravity tractors, and nuclear deflection methods for extreme cases. The Palermo Scale assesses asteroid impact risk based on size, speed, and collision probability, with over 90% of large hazardous asteroids now tracked.

Near-Earth Objects (NEOs) are asteroids and comets that pass within 48 million km of Earth's orbit. NASA identifies NEOs that could potentially impact Earth, with most known NEOs posing no significant threat in the near future. The largest known threat is asteroid 2002 MN, with a 1 in 714 chance of impacting Earth in 2000. Scientists are developing planetary defense strategies, including using spacecraft gravity to gradually redirect asteroids away from Earth's orbit, or direct impact methods if time is insufficient. While no one hopes these methods would ever be necessary, having these capabilities ensures humanity can respond to potential cosmic threats.

NASA's Planetary Defense Coordination Office addresses near-Earth object (NEO) impact hazards through systematic detection and risk assessment. Potentially Hazardous Objects (PHOs) are defined as NEOs whose orbits predict them to come within five million miles of Earth's orbit and are large enough (30-50 meters+) to cause significant damage. Risk assessment involves detecting objects using powerful telescopes, calculating trajectories to identify potential threats, and categorizing risks. The Chelyabinsk meteor demonstrated that objects of this size (30-50 meters) can cause significant damage when they unexpectedly enter Earth's atmosphere. Larger objects (over 1 km) pose greater threats but are much rarer, with impact frequencies decreasing exponentially as object size increases.

A Near-Earth Object (NEO) is any space object within 193 million kilometers of Earth—greater than the Earth-Sun distance. NASA and other agencies monitor approximately 28,000 asteroids using systems like ATLAS, which scans the night sky daily. Of nearly 30,000 NEOs, only 25 have orbits completely within Earth's orbit. Scientists have developed technology to deflect asteroid orbits if detected early enough. The DART mission was the first planetary defense system tested, and China is developing similar systems. If an asteroid is discovered 15 years before potential collision, spacecraft can impact it to change its trajectory. No known NEOs will collide with Earth in the next 100 years.

NASA's planetary defense system monitors Near-Earth Objects (NEOs) through the Center for Near Earth Object Studies (CNEOS), which maintains precise orbital catalogs tracking complete paths, future approaches, and impact probabilities for 100 years. An asteroid is classified as Potentially Hazardous (PHA) if it approaches within 0.05 astronomical units and exceeds 140 meters in diameter. Small, dark asteroids pose particular danger as they remain nearly invisible until dangerously close. Initial orbital data can indicate anything from safe flybys to potential impacts, with risk assessments constantly updated as new observations refine trajectories. Despite advanced technology, detection remains imperfect—certain regions cannot be continuously monitored, and NASA admits they do not know exactly how many hazardous objects exist.

The 2010s brought increasingly sophisticated sample return missions. JAXA's Hayabusa2 (completed 2020) successfully returned samples from asteroid Ryugu, revealing it formed in extreme heat above 1,000°C and broke from a larger parent body. The mission discovered carbonated liquid water inside iron sulfite crystals—a 'soda-like' substance containing salts and organic matter. NASA's Osiris-Rex then achieved an unprecedented 250-gram sample collection from asteroid Bennu, 50 times heavier than any previous asteroid sample. Both missions confirmed asteroids are rubble piles from ancient collisions, not solid rocks, and demonstrated that spacecraft can sink into asteroid surfaces as if they were dust.

Two major missions are returning asteroid samples to Earth: Hayabusa 2 (JAXA) landed on Ryugu in February 2019 after a 3.5-year journey, collecting surface material by firing projectiles and planning explosive cratering for deeper samples. OSIRIS-REx (NASA) visited Bennu in December 2018, using moment of inertia measurements on its sample arm to quantify collected material. Both missions compare near-Earth asteroids to understand solar system formation, with samples returning in 2020 and 2023 respectively.

Two major space missions have visited asteroids to study their composition: Japan's Hayabusa2 mission visited asteroid Ryugu in 2018 and returned samples to Earth, while NASA's OSIRIS-REx visited asteroid Bennu. Both missions represent humanity's efforts to understand the origins of life by studying extraterrestrial materials. These sample-return missions allow scientists to analyze asteroid composition in terrestrial laboratories, providing unprecedented insights into the chemical makeup of our solar system's early history.

Two major missions are collecting asteroid samples this year: NASA's OSIRIS-REx and Japan's Hayabusa 2. OSIRIS-REx will study asteroid Bennu for one year, create a precise map to select the sampling site, and return samples by September 2023. Hayabusa 2 has arrived at asteroid Ryugu and faces challenges with its laser altimeter, which cannot accurately measure altitude over the dark, carbon-rich surface. Despite this, Ryugu's mass of 450 million tons provides sufficient surface gravity (1/81 billionth of Earth's) for lander operations. The mission aims to deploy a rover and French-German lander, demonstrating international cooperation in space exploration.

Two major sample return missions are currently exploring asteroids: Japan's Hayabusa2 (launched 2014) reached asteroid Ryugu in June 2018 and successfully collected surface samples in February 2019; NASA's OSIRIS-REx (launched 2016) arrived at asteroid Bennu in December 2018. Both missions aim to collect pristine samples to study organic matter, water, and potential molecular precursors to life. These samples will help scientists compare asteroid compositions and understand how asteroids may have delivered life's building blocks to early Earth.